This invention relates in general to memory integrated circuits. In particular, this invention relates to a method and apparatus for reducing power consumption during bit line selection in memory circuits.
Memory circuits such as dynamic random access memories (DRAMs) are generally made up of a large number of memory cells arranged in the form of a matrix or array with rows and columns. FIG. 1 is a simplified block diagram of a conventional DRAM. In this typical example, memory access to the DRAM usually takes place as follows. The address buffer first reads the row address and then the column address. The addresses are passed to their respective decoders for decoding. Once decoded, the memory cell addressed outputs the stored data, which is amplified by a sense amplifier and transferred to a data output buffer by an I/O gate.
The central part of the DRAM is the memory cell array 100, which is where the data are stored. FIG. 2 is a simplified block diagram of a conventional DRAM showing an illustrative structure of the memory cell array 100. The memory cell array 100 is made up of many unit memory cells, each of which is usually individually addressable and used to store a bit. Unit memory cells are defined by word lines WLx (or rows) and bit lines BLx (or columns). The unit memory cell has a capacitor which holds the data in the form of electrical charges, and an access transistor which serves as a switch for selecting the capacitor. The transistor""s gate is connected to the word line WLx. The source of the access transistors are alternately connected to the bit lines BLx. At this level, memory access begins when a word line is selected (via the decoding of a row address) thereby switching on all the access transistors connected to that word line. In other words, all the unit memory cells in that particular row are turned on. As a result, charges in the capacitor within each unit memory cell are transferred onto the bit lines causing a potential difference between the bit lines. This potential difference is detected and amplified by a sense amplifier. This amplified potential difference is then transferred to the I/O gate activated based on the column address, which in turn transfers the amplified signal to the data output buffer.
The precharge circuit plays a significant role in detecting memory data during the course of a memory access operation. In advance of a memory access and the activation of a word line, the precharge circuit charges all bit line pairs up to a certain potential which usually equals to half of the supply potential, that is, Vdd/2. The bit line pairs are short-circuited by a transistor so that they are each at an equal potential. The precharging and potential equalization by the precharging circuit is important due to the disparate difference in capacitance between the bit lines and the storage capacitor. Since the capacitance of the storage capacitor is far less than that of the bit lines, when the storage capacitor is connected to the bit lines via the access transistor, the potential of the bit line changes only slightly, typically by 100 mV. If the storage capacitor was empty, then the potential of the bit line slightly decreases; if charged, then the potential increases. The activated sense amplifier amplifies the potential difference on the two bit lines of the pair. In the first case, it draws the potential of the bit line connected to the storage capacitor down to ground and raises the potential of the other bit line up to Vdd. In the second case, the bit line connected to the storage capacitor is raised to Vdd and the other bit line decreased to ground.
Without the precharging circuit, the sense amplifier would need to amplify the absolute potential of the bit lines. However, because of the relatively small potential change between the bit lines, the amplifying process would be much less stable and unreliable.
It should be noted that as the access transistors remain on by the activated word line, the accessed data are written back into the memory cells of one row. Therefore, the accessing of a single memory cell simultaneously leads to a refreshing of the whole word line. After the data output is completed, the sense amplifiers and the row and column decoders are disabled and the I/O gate block is switched off. At that time, the bit lines are still on the potentials according to the accessed data. The refreshed memory cells along the same row are disconnected from the bit lines by the disabled word line. The precharge circuit is activated to lower and increase respectively the potentials of the bit lines to Vdd/2 and equalize them again. The memory array is then ready for another memory access.
In addition, as previously mentioned, the data are stored in the form of electrical charges in the storage capacitor. Ideally, the charges in the storage capacitor should remain indefinitely. However, as a practical matter, the storage capacitor discharges over the course of time via the access transistor and its dielectric layer thereby losing the stored charges and the represented data. Hence the storage capacitor must be refreshed periodically. As discussed above, during the course of a memory access, a refresh of the memory cells within the addressed row is automatically performed. As is commonly known in the art, three refresh methods are typically used, namely, the RAS-only refresh, the CAS-before-RAS refresh, and the hidden refresh.
Due to physical constraints, the size of a memory array 100 is limited. Thus, in order to increase memory capacity, memory arrays 100 are typically stacked together to provide for the desired capacity. FIG. 3 is a simplified block diagram showing a typical structure having stacked memory arrays 100. The sense amplifiers 102 are shared by adjacent pairs of memory arrays 100 but otherwise perform the same function as mentioned above. The precharge circuit (not shown) which performs the precharge and equalization functions as mentioned above may be incorporated into a sense amplifier.
Referring to FIG. 3, a number of stacked memory cell arrays 100 (xe2x80x9cMCAsxe2x80x9d) are used to provide data storage. As is commonly known in the art, the number of MCAs to be used depends on the desired memory capacity and other system constraints. In FIG. 3, three representative MCAs 100a, 100b, 100c are shown. Each MCA 100 has pairs of bit lines, for example, bit line pair bl(0) and bl({overscore (0)}), accessible on its either side.
Positioned between an adjacent pair of MCAs, such as MCAs 100a, 100b and MCAs 100b, 100c, are a row of bit-line sense amplifiers 102 (each a xe2x80x9cBLSAxe2x80x9d). The number of BLSAs 102 corresponds to the number of bit line pairs of each MCA 100. Each BLSA 102 is electrically connected to both members of the adjacent pair of MCAs such as MCAs 100a, 100b. More specifically, each BLSA 102 is coupled to a bit line pair, for example, bl(0) and bl({overscore (0)}) of a MCA 100 via two switches, such as transistors 104. Hence, each BLSA 102 is connected to four transistors 104, in total, two transistors for each MCA 100.
A bit-line select controller 106 (xe2x80x9cBLSCxe2x80x9d) is used to control the operation of each row 108 of BLSAs 102. Each BLSC 106 has two control lines 110a, 110b extending therefrom. One control line 110a is connected in parallel to the gate of all the transistors 104 connecting the row 108u of BLSAs 102 to one member MCA 100a of the adjacent pair, while the other control line 110b is similarly connected to all the transistors 104 connecting the row 108u of BLSAs 102 to the other member MCA 100b of the adjacent pair.
FIGS. 4a-c are various voltage level diagrams. Specifically, FIG. 4a shows the voltage level of successively activated word lines within one MCA(i) 100b. For each active cycle, the potential of each word line, for example, WL(n), first goes from ground to Vpp and then back down to ground before the next active cycle begins. The voltage Vpp is a boosted voltage, that is greater than the internal power supply voltage Vdd used for the memory cell operation, needed to overcome the transistor threshold voltage Vt drop.
FIG. 4b shows the respective voltage levels of various BLS control lines 100a, 110b including BLS_up(i), BLS_down(i), BLS_up(i+1) and BLS_down(ixe2x88x921) during active cycles when MCA(i) 100b is activated. In this embodiment, all the BLS control lines 110a, 110b are initially at Vdd. For each active cycle, the potential of BLS_up(i) 100b and BLS_down(i) 110a for the selected array MCA(i) 100b first goes from Vdd to Vpp and then back down to Vdd before the next active cycle begins; while the potential of BLS_up(i+1) 100b and BLS_down(ixe2x88x921) 110a for the non-selected adjacent arrays MCA(i+1) 100c and MCA(ixe2x88x921) 10a first drops from Vdd to ground and then back up to Vdd before the next active cycle begins. In this way, the two rows of shared bit-line sense amplifiers 108u and 108d are connected to array MCA(i) 100b and disconnected from adjacent arrays MCA(i+1) 100c and MCA(ixe2x88x921) 100a during each active cycle.
FIG. 4c similarly shows the respective voltage levels of the same BLS control lines 110a, 110b using different relative voltage levels. In this embodiment, all the BLS control lines 110a, 110b are initially at Vpp. For each active cycle, the potential of BLS_up(i) 110b and BLS_down(i) 110a remains the same; while the potential of BLS_up(i+1) 110b and BLS_down(ixe2x88x921) 110a first drops from Vpp to ground and then back up to Vpp before the next active cycle begins.
As FIGS. 4b and 4c show, during the activation of word lines within one activated MCA(i) 100b, at least two, if not more, of the involved BLS control lines BLS_up(i) 110b, BLS_down(i) 110c, BLS_up(i+1) 110d and BLS_down(ixe2x88x921) 110a have to be switched back and forth between the designated high and low voltage levels during each active cycle. Such constant switching of the BLS control lines 110a-d during each active cycle consumes power. This type of power consumption is a particular cause for concern in modern integrated circuits. Since modern integrated circuits generally contain a high number of memory cell arrays and bit-line select controllers, the cumulative power consumption due to the constant switching of BLS control lines may reach an undesirably excessive level. Therefore, there is a need to minimize the amount of BLS control lines switching thereby reducing power consumption in memory circuits.
In accordance with the present invention, the state of a bit-line select control line connected to a currently active memory cell array is changed only when the next memory operation involves an adjacent memory cell array sharing the same row of bit-line sense amplifiers connected to such bit line select control line. Otherwise, the state of the bit-line select control line is not switched even when the array to which it connects is no longer active.
An exemplary embodiment of the present invention includes a memory circuit having a number of memory cell arrays and a number of rows of bit-line sense amplifiers. Each row of the bit-line sense amplifiers is disposed between and coupled to a pair of adjacent memory cell arrays. The exemplary embodiment further includes a number of bit-line select controllers. Each bit-line select controller includes a bit-line select control line extending therefrom and coupling to one of the rows of bit-line sense amplifiers for controlling the operation thereof. Each bit-line select control line is used to control the coupling between a row of bit-line sense amplifiers and one member of a pair of adjacent memory cell arrays. Once switched on, the bit-line select control line is switched off only when the other member of the pair of adjacent memory cell arrays is to be activated.
Another exemplary embodiment of the present invention further includes a method of operating a memory circuit. The method includes the steps of disposing a number of bit-line sense amplifiers between a pair of adjacent arrays of memory cells; activating one member of the pair of adjacent arrays of memory cells; coupling the bit-line sense amplifiers to one member of the pair of adjacent arrays of memory cells by turning on a number of coupling switches; and keeping the coupling switches on until the other member of the pair of adjacent arrays of memory cells is activated.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements and letters at the end of reference numbers are used for ease of reference to further differentiate each of a number of identical or functionally similar elements.